Given the growing environmental, economic, and governmental concerns for building new power generation facilities, utilities continue to look for cost-effective ways to defer new power generation while meeting their customers' growing demand for electricity. In this environment, power producers need to mix and match their electrical services offering to meet the customers' changing requirements. While some major improvements (such as the introduction of grain-oriented core steel) have occurred in transformer technology from time to time, other developments in the areas of core, winding, insulation, and dielectric liquids have provided only incremental improvements in transformer technology. Thus, there is a continuing need for sophisticated transformer devices that can employ modern power electronics to improve transformer functionality.
There are two possible approaches for realizing such sophisticated devices with modern power electronics: the “hybrid” design and the “all-solid-state” design. The “hybrid” design is based on the integration of a conventional transformer with power electronics only on the secondary side of the transformer. The “all-solid-state” design, on the other hand, provides a fundamentally different and more complete approach in transformer design by using power electronics on the primary and secondary sides of the transformer. The power electronics on the primary side of the transformer provide a high voltage interface with the utility Alternating Current (AC) system and the power electronics on the secondary side of the transformer provide a low voltage interface with consumer applications.
All-solid-state (electronic) transformer technology can provide control over the shape and amplitude of output voltage waveforms and can, therefore, address many power quality problems. Electronic transformer designs can solve some shortcomings found in conventional transformer technology, such as voltage drop under increasing load, “flat topped” voltage under saturation, harmonic sensitivity, containment requirements for oil spill, limited performance under Direct Current (DC) offset load unbalances, providing options for high-frequency AC, ability to convert single-phase service to three-phase for powering certain types of equipment, provide reactive compensation and so forth. In addition, this technology has the potential to lend itself to standardization of distribution transformers and to achieving other operational benefits like reduced weight and size, and reduced environmental concerns (e.g., by eliminating oil in the transformer, etc.).
One problem associated with all-solid-state transformers is the inability to provide isolation between the primary and secondary sides of the transformer. To convert two different voltage levels, it is often desirable to have transformer isolation to fully use the semiconductor switches in the transformer. One proposed solution is to modulate the input AC waveform by a power electronic converter to a high frequency square wave, which is then passed through a small, high-frequency transformer.
Several designs for solid-state power converters having high-frequency AC transformers have been proposed in the past. Some of those proposed structures can be used as building blocks for larger system structures. For example, a system structure can include multiple solid-state building blocks or modules having their inputs connected in series and their outputs connected in parallel. While such designs have some advantages (e.g., harmonic elimination, transformer isolation, reduction in size of magnetic materials, etc.), there are several drawbacks as well. For example, a problem inherent in such designs is the difficulty of ensuring that the input voltages balance among the different modules in the system structure. With device mismatching and without any active control, the input voltages among the different modules are unlikely to be maintained at the same voltage level. One solution may be to add a set of voltage balancing zener diodes, Metal Oxide Varistors (MOVs) or other passive voltage clamping methods. However, a typical passive voltage balancing element or clamping circuit consumes a large amount of power and is not practical in high-power applications.
Therefore, what is needed is multilevel converter-based intelligent, universal transformer that can interface directly to a power distribution system. The universal transformer should allow for the series connection of an unlimited number of modern power semiconductor devices while maintaining proper voltage balance.